1. Field of the Invention
The invention relates to a switch circuit, more particularly to a linearity-adjustable switch circuit.
2. Description of the Related Art
A conventional switch circuit for relaying differential voltages includes a pair of metal oxide semiconductor (MOS) switches, each of which has an input terminal and an output terminal. Each of the MOS switches can be a NMOS switch, a PMOS switch or a CMOS switch. The MOS switches receive a pair of differential input voltages (Vin+, Vin−) at the input terminals thereof, and output a pair of differential output voltages (Vout+, Vout−) at the output terminals thereof when the MOS switches conduct.
FIGS. 1 and 2 illustrate relevant simulation results of a conventional switch circuit. The horizontal axis represents a difference (Vdiff) between the differential input voltages (Vin+, Vin−), and the vertical axis represents a ratio of differential mode resistances (Rdiff) of the MOS switches (based on Rdiff acquired when Vdiff=0). Curve 21 is obtained when the MOS switches are NMOS switches or PMOS switches. Curve 22 is obtained when the MOS switches are CMOS switches. The definitions of Vdiff and Rdiff are as follows:Vdiff=Vin+−Vin−,Rdiff=(Vin+−Vin−)/(Iin+−Iin−),
where (Iin+, Iin−) are a pair of differential input currents received at the input terminals of the MOS switches.
As shown in FIGS. 1 and 2, differential mode resistances (Rdiff) of the MOS switches change according to the difference (Vdiff) between the differential input voltages (Vin+, Vin−). As such, when the switch circuit is applied to a linear circuit (such as a filter or an amplifier), linearity of the linear circuit will be restricted.